The present invention relates generally to the connection or bonding of integrated circuit dies to metal lead frames prior to encapsulation of the dies. More particularly, the invention relates to tape automated bonding of the dies of the lead frame with an improved copper tape treated to resist electromigration.
In the packaging of semiconductor devices, it is necessary to interconnect the device, generally referred to as a die or chip, to a plurality of electrical leads formed on a metal lead frame. Such interconnection is generally accomplished by one of two distinct techniques, referred to as wire bonding and tape automated bonding. Wire bonding is a sequential process where wire bonds are formed individually between bonding pads on the die and the inner ends of the leads on the frame. While the wire bonding is the method of choice for some applications, it is relatively slow since it requires that the bonds be formed one after the other. Wire bonding a die can require several seconds to complete even with the most rapid automatic wire bonding equipment.
Tape automated bonding, in contrast, allows the formation of all connections between the die and the lead frame to be made simultaneously. A metal tape, typically copper, is fabricated by conventional stamping and etching techniques so that a plurality of individual leads, often referred to as beams, are formed. The beams are cantilevered from the tape. That is, they remain connected to the tape at their outer ends, while their inner ends are free to allow connection to the die. By forming the inner ends of the beams in a pattern which corresponds precisely to the pattern of metallization pads formed on the die, the inner ends of the beams can be aligned with the die and then connected in a single compression and heating step. In a similar operation, the outer ends of the beams are connected to the lead frames, and the beams then severed from the remainder of the tape.
In connecting the inner ends of the lead beams to the semiconductor die, it is necessary to form a bump on either the die or the beam. The bump, which is formed from metal, provides both the material necessary for forming the connection between the beam and the die, as well as providing for a short distance or offset between the beam and the die. There are advantages and disadvantages associated with forming the bump on either the semiconductor die or the beam. The present invention is directed at bumped bonding tapes, that is bonding tapes having a connection bump formed at their inner ends.
It has been found by the inventors herein that when employing copper tape for tape automated bonding, problems can arise with electromigration of the copper on the surface of the die. Such electromigration results from the potential difference between the metallization pads on the die which results in dendritic flow of the copper on the die surface. In the worst case, the copper can migrate and form a short between two or more of the metallization pads, rendering the device defective.
It would thus be desirable to provide an improved copper bonding tape which would avoid the problems associated with copper migration on the surface of a die. It would be particularly desirable if the method for forming the improved tape would require only small departures from existing techniques for copper tape fabrication.